In semiconductor integrated circuits (ICs), a resistor may be used to control the resistance of other electronic components of the IC. As is known to those skilled in the art, the resistance, R, of a resistor is proportional to the length, L, of the resistor and the reciprocal cross sectional area, 1/A, of the resistor; the L and A are measured in the direction of current flow. The basic equation for resistance of a resistor is thus: R=L/A, where R, L and A are as defined above.
Prior art resistors are typically composed of polysilicon that has been doped. As the integration of semiconductor devices increases, each component within a semiconductor IC has to provide equivalent or better electrical properties. A downscaled resistor thus has to provide a constant resistance value that does not fluctuate much during use. However, due to the properties of polysilicon, a prior art resistor comprised of doped polysilicon can only provide a limited resistance within a limited space. Employing a polysilicon resistor to provide relatively high resistance then becomes a problem in designing and fabricating a highly integrated semiconductor device.
Recently, doped polysilicon resistors have been replaced with a sinlge thin film resistor that is comprised of a material that has a higher resistivity than that of polysilicon. Examples of such higher resistivity materials include, but are not limited to: TiN and TaN. Tantalum nitride, TaN, containing 36% N2 is a material currently being used in the back-end-of-the line (BEOL) of most semiconductor devices. Even though higher resistivity materials can be used to fabricate good resistors, they typically exhibit a very high temperature coefficient of resistivity, i.e., TCR, that is on the order of about −600 ppm/° C. TCR, which is the normalized first derivative of resistance and temperature, provides an adequate means to measure the performance of a resistor.
On account of the high TCR values of prior art single thin film resistors, the resistance of such resistors tends to fluctuate a lot when the resistor is used at normal operating temperatures of about 85° C; resistance fluctuation hampers the performance of high-performance semiconductor IC devices. For example, if a resistor having a resistivity of 50 ohms is provided in a semiconductor IC, high TCR of the resistor may cause the resistance to vary as much as 15 to 20% from the desired resistance of 50 ohms as it is being used and heated up via Joule heating. As such, the 50-ohm resistor is not operating at the resistance value it was intended to operate at.
In view of the state of the art mentioned above, there is a need for providing new and improved resistors that have a targeted sheet resistance and a TCR value that is substantially zero. The term “substantially zero” is used in the present invention to denote a TCR value that is within ±50 ppm/° C from zero.